At the present time, many functionalities are associated with the various type of lighting devices. Amongst these functionalities there are for example a capacity for movement for certain lights, in particular used when the motor vehicle is taking a bend, to illuminate in an optimum fashion the path followed by the vehicle in the bend; there are also found an ability of certain dual-mode headlights to switch automatically between the use of the dipped beams and the use of the main beams, in order in particular to avoid dazzling the drivers being passed. There is also found an ability of certain light devices, usually the dipped lights, to switch on automatically when the external luminosity conditions require a switching on of the lights, in particular when the motor vehicle has entered a tunnel.
The solutions existing for the latter functionality are essentially based on the use of photodiodes. The latter make it possible to detect an external luminosity level; a tunnel, even illuminated by artificial light sources, does not have a luminosity comparable with that of the natural light in daylight. The use of a single photodiode therefore makes it possible to cause the switching on of the headlight devices once the vehicle has entered the tunnel.
However, it is impossible with such equipment to anticipate the switching on of the lights so that the vehicle arrives in the tunnel with its headlight devices already switched on. Such a solution for automatic switching on of the lights, based on the use of a single photodiode, therefore poses a certain number of problems. This is because the fact that it is not possible to anticipate the switching on of the lighting devices when arriving in a tunnel has the consequence that the driver, on entering the tunnel in question, has reduced visibility until the lighting devices are switched on, a switching on which, typically, takes a second, the conventional reaction time of a photodiode. However, the travel time of one second corresponds, for an average speed of 90 kilometers per hour, to a distance travelled of 25 meters, a not insignificant distance under non-optimal visibility conditions. Moreover, when the lighting devices which are switched on use light sources of the xenon type, it is necessary to add to the reaction time of the photodiodes a stabilization time for the light flux emitted by the lighting device before having satisfactory illumination available.
In the prior art, a solution was proposed for causing early switching on of the lighting devices, the switching on occurring before the vehicle enters the tunnel in question. Such a solution involves a set of three photodiodes: a first photodiode points towards the sky; a second photodiode points towards the front of the vehicle, and therefore in certain cases towards the tunnel; and a third photodiode points towards an intermediate region. The appearance of a tunnel facing the vehicle then results in characteristic signals generated by the three photodiodes. However, the characteristic signals thus generated can be interpreted as the imminent arrival in a tunnel only if these signals coincide with a known template, which must be stored in advance; however, because of the great variety existing in the forms of tunnel entrance, it is then essential to store an enormous amount of information, in order to take account of as many templates as there exist different forms of tunnel entrances. Moreover, the directivity of the photodiodes does not make it possible, or makes it possible too late, to allow early switching on of lighting devices, to detect a tunnel placed at the exit from a bend. Finally, equipping a vehicle with equipment dedicated to the early detection of tunnels—two additional photodiodes compared with normal equipment limited to a single photodiode—represents a not insignificant cost.